1) Field of the Invention
This invention relates to integrated circuits and semiconductor devices. It relates particularly to a structure and method for producing integrated circuits having improved moisture and contamination guard rings surrounding openings in insulating layers, such as fuse openings.
2) Description of the Prior Art
Fuses can be used to rewire memory and logic circuits. For example, in dynamic or static memory chips, defective memory cells may be replaced by blowing fuses associated with the defective cells, and activating a spare row or column of cells. This circuit rewiring using fusible links allows considerable enhanced yields and reduces the production costs. Also, logic circuits may also be repaired or reconfigured by blowing fuses. For example, it is common to initially fabricate a generic logic chip having a large number of interconnected logic gates. Then, in a final processing step, the chip is customized to perform a desired logic function by disconnecting the unnecessary logic elements by blowing the fuses that connect them to the desired circuitry. Still other applications of laser-blown fuses are possible.
An important challenge is to improve the reliability and yields of the semiconductor devices surrounding openings in insulating layers, such as openings over fusible links. A problem with openings is that moisture and other contaminants can diffuse from the openings into the device areas thus reducing circuit reliability and yields.
FIG. 1A shows a top plan view of a semiconductor chip 82 with openings 84 through the insulating layers called fuse (cutting) openings 84. Also, a semiconductor chip 82 sometimes contains openings over alignment marks which are used to align the laser repair machine and other tools.
FIG. 1B shows a conventional fusible link region and an adjacent device region in a top down view. FIG. 1C shows a cross-sectional view of the same link and device regions taken along horizontal axis labeled 1C in FIG. 1B. Fuse 86 can be formed of a metal.
Fuse 86 is often formed over thick field oxide regions 88 in semiconductor substrate 10 as shown in FIG. 1c. Fuse 86 is formed over the field oxide regions 88 to prevent shorting of the fuse 86 to the substrate 10 through a thinner insulating layer. Layers 90, 92, 94, are insulating layers. Opening 84 is formed over the fuse area through the insulating layers 90, 92, 94. An adjacent semiconductor device is shown with buried doped regions 100 106, gate oxide 102, gate 104, via 110 and metal layers 108, 112. The fuse 86 is shown in FIG. 1c with a hole 83 formed after the fuse was "blown" (i.e., cut or heated) by a laser. Contamination can diffuse through the hole 83 into the field oxide layer 88 and then into the other insulating layers 90, 92, 94 to the devices 100, 102, 106.
There are two methods for blowing fuses: (a) using a laser and (b) passing a high current through the fuse. The portion of the fuse and thin insulating layer which is melted away or "blown" should not deposit on or interfere with near-by devices.
A laser is often used to break the fuse forming an electrical open by heating the fuse to a high temperature. It is conventional to have an opening 84 over the fuse in the area where the fuse will be broken so that the laser heating will be more effective. In addition, openings are often formed over alignment marks which are used to align the laser on the correct portion of the fuse to be blown. The alignment mark openings in the passivation layers are formed so that the alignment marks can be clearly viewed.
A major problem with any window opening in the passivation layers is that moisture and contamination can enter through the exposed insulation layers and diffuse to the semiconductor devices. The diffused moisture and contaminates can decrease reliability and yields. Moisture is present in the air and sodium (Na+ions) is plentiful in the environment.
As shown in FIG. 1C, moisture and other contaminants can enter through the hole 83 into layer 88 and diffuse to the adjacent semiconductor devices. Water will attack the metal via 110, with the following reaction: EQU 3H.sub.2 O+Al.fwdarw.Al(OH).sub.3 +3/2H.sub.2
The moisture corrodes the metal causing the resistance of metal via's 110 to increase and finally cause circuit failure. Mobile ions, such as sodium ions, can diffuse through insulating layers 90, 92, 94 and field oxide layer 88. Mobile ions in the field oxide layer 88 can also cause field inversion. The field inversion causes undesired leakage current between adjacent buried doped regions 100. Also, mobile ions in the gate oxide 102 will cause a transistor threshold shift whereby the circuit falls. Furthermore, moisture can cause the insulating layers to delaminate causing circuit failure.
The following U.S. patent shows a fusible link structure, but do not solve the problem of moisture diffusing through the window opening and the insulating layers. Takayama et at., U.S. Pat. No. 4,536,949, discloses a method of forming an opening in insulating layers over a fuse. The opening is formed by etching each insulating layer over the fuse after the insulating layer is formed. However, this does not solve the problem of contaminates entering the fuse opening and defusing to the adjacent semiconductor devices.